From JP 64-21826 U, JP 09-060666 A2, and JP10292834, the use of a gasket acting on the piston as a retracting member thereof is known.
This gasket is received in a cylinder surface seat and acts by pressing on the piston so as to be able to strain itself in the piston moving direction, while remaining with one surface in contact with the piston while this moves by being urged by the brake fluid pressure. As the brake fluid stops urging the piston, the gasket elastically and axially strained by the piston returns to the original shape thereof, thus dragging the piston into the cylinder.
In the case where the pad affected by the piston wears out, during the braking action, beyond the gasket strain limit, the piston begins to slide relative to the strained gasket, thus compensating the pad wear amount. Once the braking action is completed, the piston is returned by the gasket only by the amount that is proportional to the strain thereof.
These known piston retracting devices, even if they are satisfactory under many points of view, have an inherent functioning variability related to the caliper body strain when it is subjected to high braking pressures exerted on the disc opposite friction surfaces. In fact, the caliper body, which is arranged astride the disc, with portions thereof facing opposite friction surfaces of the same disc, reacts to the pressure action exerted by the piston(s) by moving away from the disc, initially in an elastic manner, and then, in the case of an elastic strain, it comes back to the position thereof as the braking action stops. This movement away and towards the disc of the caliper body occurs independently from the piston relative movement relative to the seat thereof of the caliper body.
Further, this movement results, as the braking action which cause the caliper body portions to approach the disc friction surfaces stops, in an approach of the piston from the friction surfaces which is not caused by the relative movement between the piston and the seat thereof, however changing the piston relative position relative to the disc. Therefore, this piston position is determined not only by the limited elastic return of the gasket, but in some cases also by the recovery of the caliper elastic strain which, by returning the piston to the proximity of the disc, exerts a thrust of the piston on the pads and the creation of a residual braking torque also when the driver's intentional action stops.